Communication devices such as wireless devices are also known as e.g., terminals, User Equipments (UE), mobile terminals, wireless terminals and/or mobile stations. Any reference herein to a UE may be understood to refer to a wireless device, or vice versa. Wireless devices are enabled to communicate wirelessly in a cellular communications network or wireless communication system, sometimes also referred to as a wireless communications network, cellular radio system or cellular networks. The communication may be performed e.g. between two wireless devices, between a wireless device and a regular telephone and/or between a wireless device and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
Wireless devices may further be referred to as mobile telephones, cellular telephones, laptops, or surf plates with wireless capability, just to mention some further examples. The terminals in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another terminal or a server.
The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area being served by an access node such as a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, “B node”, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g., macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the terminals within range of the base stations. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the mobile station. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the mobile station to the base station.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
3GPP LTE radio access standard has been written in order to support high bitrates and low latency both for uplink and downlink traffic. All data transmission is in LTE controlled by the radio base station.
In LTE, voice services are not necessarily supported, it depends on network configuration. Therefore, in LTE, which uses the Evolved Universal Terrestrial RAN (E-UTRAN), if a wireless device needs to make or receive a call, and if voice services are not supported in LTE, a communication with the wireless device is transferred from the LTE network to a 2nd Generation (2G), through e.g., a Global System for Mobile communications (GSM)/Enhanced Data rates for GSM Evolution (EDGE) RAN (GERAN)) or a 3G (through e.g., a Universal Terrestrial RAN (UTRAN), network. This 3GPP specified procedure is known in as a Circuit Switched Fall Back (CSFB), since the communication returns to LTE after the Circuit Switched (CS) call. Once the call is finished, the UE may be transferred back to the LTE network when the radio resources are released. If an LTE cell providing coverage cannot be found when returning a communication with the UE to LTE from a 2G or 3G network, service outage time increases.